Question

A convex lens of focal lenght 18 cm & a concave lens of focal lenght 24 cm are placed in contact such that they have a common principal axis. Will the combination act as a convex lens or a concave lens? Find the focal lenght & power of the combination.

Answer 1

Focal length of convex lens

f = 18 cm

Focal length of concave lens
f' = -24 cm

Combined focal length of lens combination is given by

= f+ f'

= 18-24

= -6 cm = 0.06 m

Negative sign shows that combination of lens is concave lens.

Power of lens combination is given by

p = 1/focal length

= 1/(0.06)

=100/6 =50/3

=16.67 D